"We understand in our state that part of our unfinished business is to help other states in the United States of America achieve marriage equality," Quinn said, saying it was important to ensure that "love is not relegated to a second class status to any citizen in our country."

As The Associated Press writes:

"The road to the Illinois vote was long and included a stalled attempt earlier this year, frustrating activists in a state where Democrats lead the House, Senate and governor's office. Chicago Democratic Rep. Greg Harris, who is the main sponsor, decided not to bring the bill for a vote in May, saying he didn't have the support.

"Then the U.S. Supreme Court ruled to strike down a provision of the federal Defense of Marriage Act, which Harris said resonated with lawmakers. Backers also launched a furious campaign, hiring a union lobbyist, the former head of the Illinois Republican Party and field organizers statewide."

The measure had the backing of both of Illinois' U.S. senators as well as Chicago Mayor Rahm Emanuel.

Illinois follows Hawaii, whose Gov. Neil Abercrombie signed similar legislation last week. Illinois is the 15th state to approve same-sex marriage. The District of Columbia has also approved such unions.